1. Field of the Invention
The present invention relates to a duplex printer that prints images on both surfaces of a sheet of paper. More particularly, the present invention relates to a photoelectrographic printer for duplex printing, which uses a paper conveying belt for conveying the paper when a front surface and a back surface of the document are printed.
2. Description of the Related Art
In general, a printer using an electrophotographic method is a device that forms an electrostatic latent image by irradiating light on a photosensitive medium charged to a predetermined electric potential, develops the electrostatic latent image with a toner of a predetermined color using a developing apparatus, and transfers and fixes the image on a sheet of paper to print a mono-color image or full-color image.
The electrophotographic printer can be classified into a wet type and a dry type electrophotographic printer according to a type of developer. The wet type electrophotographic printer uses a developer in which liquid carrier includes distributed powder toner. The dry type electrophotographic printer uses a binary developer including a powder type carrier and a toner or a unitary developer including the toner only. Hereinafter, the dry type electrophotographic printer will be described, and the developer will be referred to as toner.
In order to print a full-color image, yellow (Y), magenta (M), cyan (C), and black (K) toner colors are required, thus four developing devices that develop the four color toners are required. To form a full-color image, two methods can be used. The first is a single path method, wherein a transfer substance passes through photosensitive substances on which four color images are formed and the color images on the photosensitive substances are sequentially transferred. The second is a multi-path method, in which when a transfer substance passes through photosensitive substances, an image on the photosensitive substances is transferred from only one photosensitive substance and the transferring process is repeated to form a full-color image. In both methods, four developing devices are required.
To print a color image, the multi-path method requires a processing time four times longer than for printing a mono-color image, however, the structure of a printing apparatus is simple. On the other hand, in the single-path method, the printing time to print a mono-color image or a full-color image is same, and color printing can be performed with high speed.
FIG. 1 is a schematic view illustrating a conventional electrophotographic image forming apparatus using the single-path method.
Referring to FIG. 1, the image forming apparatus includes a light scanning unit 20 that scans light onto a photosensitive drum 10, which is charged to a predetermined electric potential, to form an electrostatic latent image, four developing devices 30 that develop the electric latent image with four color toners (Y, M, C, and K) to form toner images on the photosensitive drums 10, a feeding belt 40 that conveys a sheet of paper, on which the toner images of four colors formed on the photosensitive drums 10 are transferred sequentially, and a fusing device 50 that fixes the image onto the paper by heating and pressing the paper on which the toner images are transferred. A paper feeding cassette 60 that feeds the paper (P) is installed to be input in and output from a main body of the image forming apparatus, and the paper placed on the paper feeding cassette 60 enters a printing path by a pickup roller 70. A developing roller 31 is disposed on a front end of the developing device 30, and the developing roller 31 develops the latent image on the photosensitive drum 10. The photosensitive drum 10 and the developing devices 30 are disposed horizontally, and perform the color printing process in the single-path method to form a compact type image forming apparatus. Reference numeral 11 denotes transfer rollers corresponding to the photosensitive drums to transfer the toner images on the photosensitive drums on the paper (P). A voltage having different polarity from that of a toner is applied to the transfer rollers 11.
On the other hand, U.S. Published Application Ser. No. 2003/108359, which is incorporated herein by reference, discloses a duplex printer, in which a single printing path and a duplex printing path are formed using guide plates. In this case, a transfer process is performed in a horizontal direction, and a space for a plurality of photosensitive drums is required to perform color printing. Thus, it is difficult to construct a compact color printer. Also, the above duplex printer has a complex structure that includes upper and lower plates, which guide the paper for making the paths for printing on a back surface of a sheet of paper, a roller for conveying the paper between the plates, and a gear train for actuating the roller.
Therefore, it is desirable that the color printing is performed on a vertical plane to make a compact duplex printer, and a paper feeding belt as shown in FIG. 1 can be used.